1. Field of the Invention
This invention relates to the field of optical data transport networks.
2. Related Art
There are two predominant network architectures for creating optical data transport networks. These network architectures are commonly referred to as "ring" and "mesh."
In a ring network architecture each node of the network is connected to exactly two adjacent nodes to form a ring topology. When a break occurs in the ring, data traffic is looped back along a reverse direction spare path to bypass the break in the ring. Present day optical ring networks are implemented with devices known as add/drop multiplexers (ADMs). An ADM is an all optical switch that either chooses (or is commanded to select) either a working path or spare restoration path and switches traffic itself to these paths.
The ADM ring architecture is attractive for its simplicity and recovery speed, which is around 50 milliseconds. A drawback of the ADM ring architecture is that it is impractical to implement large networks as a single ring. Another drawback is that ADM ring networks require that there be a 1:1 protect ratio (i.e., one spare connection for each working connection) in order to implement ring recovery.
In a mesh architecture nodes of a network can be connected to more than two other nodes, and a given signal may have many possible routes by which to traverse the network. A network covering, a large geographical area is amenable to the mesh design. As the network expands, nodes can simply be added at the periphery and be connected to other nodes in any manner as needed. In addition, nodes can be easily added inside the network to provide more dense coverage of an area as needed. With respect to network restoration, the mesh architecture allows for sophisticated actions to be taken in response to a failure. For example, if a failure of several connections occurs suddenly within the network, the switches within the network can perform a coordinated switching operation to divert traffic around the failure. For this purpose, most of the spans within a mesh network are equipped with extra connections (i.e., spare connections) that are used for emergency backup when one or more working connections fail.
Whereas an ADM ring architecture requires a 1:1 protect ratio, the mesh architecture allows a considerably lower protect ratio due to the flexibility of the mesh switching. The tradeoff is that mesh restoration is more complex and more time consuming. Mesh restoration requires on the order of 1 or 2 seconds to restore a span failure.
What is needed is a network design approach that exhibits the simplicity and fast switching of an ADM ring network yet offers the spare efficiency and ease of growth characteristics of a mesh network.